RECHARGE FEASIBILITY ASSESSMENT FOR THE CITY OF PHOENIX NORTH GATEWAY WATER RECLAMATION PLANT PHOENIX, ARIZONA

The objectives of this investigation focus on how geophysical methods (both surficial and downhole) and borehole drilling methods were used to evaluate the feasibility of the vadose zone recharge well method for storing excess reclaimed water at the City of Phoenix North Gateway Water Reclamation Plant (NGWRP) site. The vadose zone recharge well method has been used throughout the Salt River Valley as a reclaimed water storage method for other sites.The City of Phoenix NGWRP will produce 4 million gallons per day (mgd) of tertiary treated wastewater during its first phase of operation. The planned ultimate treatment capacity at the NGWRP is 32 mgd. To coincide with the City of Phoenix's Long-Range Water Resources Plan, the reclaimed water will be artificially recharged using vadose zone recharge wells and reused as customers become available. This will allow the City of Phoenix to accrue long term storage credits which can be used to supplement future water resources needs.Pre-existing drilling information and the existence of volcanic outcroppings located approximately onehalf mile from the proposed reclamation plant suggested that subsurface intercalated basalt flows were pervasive within the focus area, including the plant site. A geophysical evaluation and drilling program was initiated to determine the actual character of the alluvial basin beneath the NGWRP site. The geophysical evaluation included gravity and seismic surveys to define the extent of the basin fill and estimate the formation velocity. The drilling program included the drilling of two soil borings to a depth of 300 feet bls and two monitor wells that were constructed to 350 feet bls to collect hydrogeologic data at the site and assist with evaluation of the geophysical surveys. Additionally, a small scale recharge facility was constructed at the site for testing.